The subject matter disclosed herein relates to a harmonic estimator and, more particularly, to a harmonic estimator for tip clearance in a rotorcraft.
A coaxial rotorcraft is characterized as having an airframe and a main rotor assembly at a top portion of the airframe. The main rotor assembly includes upper and lower main rotors that rotate about a common rotational axis relative to the airframe in opposite directions to generate lift and thrust for the aircraft. Tip clearance between the respective blades of the upper and lower rotors refers to the distance between the blades of the upper rotor and the blades of the lower rotor as they pass one another during the rotations at blade crossings. Generally, tip clearance needs to be maintained above a pre-defined lower limit to insure that the blades of the upper and lower rotors do not hit one another.
In order to insure that tip clearance is maintained above this lower limit, the tip clearance is typically monitored using direct sensing methods such as proximity sensors. However, since these direct measurements only measure the tip clearance at blade crossings, the direct measurements do not necessarily have predictive capability. Also, since proximity sensors only provide tip clearance measurements, they cannot be used to gather additional state information.